Messages in this thread | | | Date | Sun, 7 Nov 2010 22:40:27 +0100 | From | Frederic Weisbecker <> | Subject | Re: perf tools miscellaneous questions |
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On Thu, Nov 04, 2010 at 09:52:09AM +0100, Francis Moreau wrote: > Frederic Weisbecker <fweisbec@gmail.com> writes: > > > On Wed, Nov 03, 2010 at 08:28:59PM +0100, Francis Moreau wrote: > >> Hello, > >> > >> I'm trying to use perf-tools and also to learn some internals about > >> them. So I prefer to ask all of them in one email. > >> > >> The first one is about the list of pre-defined events given by > >> perf-list. I couldn't find any documentations that describes these > >> events so excuse me if the question is stupid. > > > > > > > > Sorry about that. We indeed need to improve a lot the documentation. > > May be this particular part could come with the future sysfs exposure > > of the events. > > > > No problem, but yes this part should be documented somewhere. And I > think the syntax of event too, specially the modifier like 'u' or 'p'.
Ah that is documented in "man perf-list".
> > >> > >> What's the difference between 'cpu-clock' and 'task-clock' event ? > > > > > > cpu-clock is based on the total time spent on the cpu. task-clock is > > based only on the time spent on the profiled task, so that doesn't count > > time spent on other tasks, it has a per thread granularity. > > Ok, so 'cpu-clock' could have been named 'proc-clock' even though a task > is a processus on Linux.
Well, this is a matter of opinion probably, I think cpu-clock defines better its role.
> > [...] > > >> The last question is about the source code annotation done by > >> perf-report. I'm using it to locate the place in my code that generates > >> the most data cache miss events. I can read this during a perf-report > >> session: > >> > >> [...] > >> 0.00 : df215: c3 retq > >> 0.00 : df216: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1) > >> 0.00 : df21d: 00 00 00 > >> 10.00 : df220: 48 8b 75 00 mov 0x0(%rbp),%rsi > >> 80.00 : df224: 48 89 df mov %rbx,%rdi > >> 0.00 : df227: 41 ff d4 callq *%r12 > >> 0.00 : df22a: 85 c0 test %eax,%eax > >> [...] > >> > >> If I read the output correctly, most of the dcache misses are coming from > >> 'mov %rbx, %rdi', and AFAIK this intruction can't generate any dcache > >> miss. What am I missing ? > > > > > > Perhaps you need pebs to get the very precise location on your event. > > > > perf stat -e cache-misses:up,l1d-loads-misses:up true > > > > > > I think the more you add 'p', the more precise it is. > > Like: > > > > perf stat -e cache-misses:uppp,l1d-loads-misses:uppp true > > > > Not sure how much it will accept though :) > > Well it doesn't want one actually: > > $ perf stat -v -e cache-misses:up true > Error: counter 0, sys_perf_event_open() syscall returned with -1 (No > space left on device) > No permission to collect stats. > Consider tweaking /proc/sys/kernel/perf_event_paranoid. > > Where can I find a description of PEB ?
I have the same problem. But running perf record with this :p works for me. Which is what we want: pebs is useful for sampling, not counting-only.
Ah and that won't work if you don't run some intel CPU I think. Check you have PEBS support in /proc/cpuinfo
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